Who must always wear a safety belt when a vehicle is in motion?

Answer: D — All of the above

All of the above must wear safety belts when a vehicle is in motion - this includes passengers in both the front and back seats, as well as children under the age of 13. This is to ensure the safety of everyone in the vehicle in case of any sudden stops, collisions, or other accidents. It is important for all passengers to wear a safety belt to prevent serious injuries and follow traffic laws.